There are several lines of evidence to suggest that the control of c-myc at the translational and posttranslational levels is critical for the molecular and biological function of c-myc. During the past project period we have found that the non-AUG-initiated c-Myc l protein is very different from the AUG- initiated c-Myc 2 protein in terms of regulation, molecular function and biological function. The synthesis of c-Myc 2 has been shown to be necessary for growth and its deregulation can lead to tumorigenesis. Also, only c-Myc 2 protein induces hyperploidy in murine erythroleukemia cells, a phenotype associated with more advanced tumorigenic cells. In contrast, c-Myc 1 protein synthesis appears to be incompatible with growth and tumorigenesis. The synthesis of c-Myc 1 is low in growing cells and becomes induced to high levels at high cell densities. Also, c-Myc 1 synthesis is lost in many lymphomas. These observations suggest that a regulated balance of these two proteins is necessary for normal growth control. To achieve the proper ratios of the two c-Myc proteins under specific growth conditions there appears to exist an unusual translational mechanism to induce c-Myc 1 synthesis which can be triggered by amino acid deprivation. In addition to differential regulation and biological properties, we have also found that the two c-Myc proteins have very different transcriptional activities through a new DNA binding sequence specific for Myc heterodimers. Finally, we have identified and characterized a cluster of interdependent phosphorylation sites in the transactivation region of the c-Myc proteins which are regulated by growth and mitogens. One of these sites is mutated in most v-Myc proteins while a new site appears to be phosphorylated by a nuclear kinase. Overall, our hypothesis is that these translational and post-translational events represent basic regulatory mechanisms which control c-myc and other similar nuclear transcription factors in culture and in the animal. To test this hypothesis this proposal will focus on the following specific aims: l) Determine the translational mechanism controlling the induction of non-AUG- initiation by amino acid deprivation in hematopoietic cells; 2) Determine whether non-AUG initiation is regulated in mice using specific myc-LacZ constructs introduced into transgenic mice; 3) Characterize novel regulated phosphorylation events of c-Myc proteins in hematopoietic cells; and 4) Determine the effects of phosphorylation on the transcriptional and biological activities of the c-Myc proteins.